Selective removal of liquid developer in a cyclical electrophotographic process

ABSTRACT

An electrostatographic imaging surface is cleaned of liquid developer by selectively applying to the charged or image areas of the imaging surface a finely divided, dry, absorbent powder. The powder may be selectively applied by charging the powder to a polarity opposite that which the imaging surface bears and presenting a charged powder loaded applicator adjacent the imaging surface such that the charged powder is transferred to the imaging surface with an electrostatic assist from the residual charge on the imaging surface.

United States Patent Mammino [4 1 Apr. 18, 1972 [54] SELECTIVE REMOVALOF LIQUID 2,937,390 5/1960 Baton et a1 ..1s/3 DEVELOPER IN A CYCLICAL2/1322 g gi n et l-m- M91 ?/4/7 ELECTROPHOTOGRAPHIC PROCESS t u m 53,129,115 4/1964 Clark et al. .118/637 [72] Inventor: Joseph Mammino,Penfield, N.Y. 3,382,101 5/1968 Bishop ..l34/7 [73] Assignee: XeroxCorporation, Rocheste 3,405,683 10/1968 Jons et a1 ..118/637 [22] Filed:Nov. 20, 1969 Primary Examiner-George F. Lesmes Assistant Examiner-JohnC. Cooper, 111 [2]] App! 878495 Attorney-James J. Ralabate, Albert A.Mahassel and Samuel E. Mott [52] U.S. Cl ..96/1.4,96/1 LY, 117/17.5 LY,134/79, 355/15 {57] ABSTRACT 51 I t. Cl. ..G03 13 22 issi Field ofSearch ..96/1.4, 1; 134/19; 252/110; electrostamgmplm surface cleaned15/3, 355/15, 117/17 5 37 developer by selectively applying to thecharged or image areas of the imaging surface a finely divided, dry,absorbent powder. The powder may be selectively applied by charging [56]References cued the powder to a polarity opposite that which the imagingsur- UNITED STATES PATENTS face bears and presenting a charged powderloaded applicator adjacent the imaging surface such that the chargedpowder is 1,1 1 1,445 9/1914 Dunton ..134/7 transferred to the imagingSurface with an electrostatic assist 319 7 from the residual charge onthe imaging surface. 1S 2,772,991 12/1956 lnsalaro ..134/7 14 Claims, 3Drawing Figures PKTENT EDAPR 1 8 I972 INVENTOR JOSEPH MAMNHNO SWJZMWATTORNEY V l SELECTIVE REMOVAL OF LIQUID DEVELOPER IN A CYCLICALELECTROPHOTOGRAPI-IIC PROCESS I BACKGROUND OF THE INVENTION Thisinvention relates to imaging systems, and more particularly, to improvedcleaning systems and techniques.

The formation and development of images on the surface ofphotoconductive materials by electrostatic means is well known. Thebasic electrostatographic process, as taught by C. F. Carlson in U.S.Pat. No. 2,297,691 involves placing a uniform electrostatic charge on aphotoconductive insulating layer, exposing the layer to a light andshadow image to dissipate the charge on the areas of the layer exposedto the light and developing the resulting electrostatic latent image bydepositing on the image a finely divided electroscopic material referredto in the art as toner. The toner will normally be attracted to thoseareas of the layer which retain a charge, thereby forming a toner imagecorresponding to the electrostatic latent image. This powder image maythen be transferred to a support surface such as paper. The transferredimage may subsequently be permanently affixed to a support surface as byheat. Instead of latent image formation by uniformly charging thephotoconductive layer and then exposing the layer to a light and shadowimage, one may form the latent image directly by charging the layer inimage configuration. The powder image may be fixed to thephotoconductive layer if elimination of the powder image transfer stepis desired. Other suitable fixing means such as solvent or overcoatingtreatment may be substituted for the foregoing heat fixing step.

Similar methods are known for applying the electroscopic particles tothe electrostatic latent image to be developed. Included within thisgroup are the cascade development technique disclosed by E. N. Wise inU.S. Pat. No. 2,618,552; the powder cloud technique disclosed by C. F.Carlson in U.S. Pat. No. 2,221,776 and the magnetic brush processdisclosed, for example, in U.S. Pat. No. 2,874,063.

Development of an electrostatic latent image may also be achieved withliquid rather than dry developer materials. In conventional liquiddevelopment, more commonly referred to as electrophoretic development,an insulating liquid vehicle having finely divided solid materialdispersed therein contacts the imaging surface in both charged anduncharged areas. Under the influence of the electric field associatedwith the charged image pattern, the suspended particles migrate towardthe charged portions of the imaging surface separating out of theinsulating liquid. This electrophoretic migration of charged particlesresults in the deposition of the charged particles on the imagingsurface in image configuration.

A further technique for developing electrostatic latent images is theliquid development process disclosed by R. W. Gundlach in U.S. Pat. No.3,084,043 hereinafter referred to as polar liquid development. In thismethod, as electrostatic latent image is developed or made visible bypresenting to the imaging surface a liquid developer on the surface of adeveloper dispensing member having aplurality of raised portions orlands defining a substantially regular patterned surface and a pluralityof portions depressed below the raised portions or valleys. Thedepressed portions of the developer dispensing member contains a layerof conductive liquid developer which is maintained out of contact withthe electrostatographic imaging surface. Development is achieved bymoving the developer dispensing member loaded with liquid developer inthe depressed portions into developing configuration with the imagingsurface. The liquid developer is believed to be attracted from thedepressed portions of the applicator surface in the charged fieldor'image areas only. The developer liquid may be pigmented or dyed. Thedevelopment system disclosed in U.S. Pat. No. 3,084,043 differs fromelectrophoretic development systems where substantial contact betweenthe liquid developer and both the charged and uncharged area of anelectrostatic latent image bearing surface occurs. Unlikeelectrophoretic development systems,

substantial contact between the polar liquid and the areas of theelectrostatic latent image bearing surface not to be developed isprevented in the polar liquid development technique. Reduced contactbetween a liquid developer and the non-image areas of the surface to bedeveloped is desirable because the formation of background deposits isthereby inhibited. Another characteristic which distinguishes the polarliquid development technique from electrophoretic development is thefact that the liquid phase of a polar developer actually takes part inthe development of a surface. The liquid phase in electrophoreticdevelopers functions only as a carrier medium for developer particles.In general the developer technique disclosed in U.S. Pat. No. 3,084,043may provide development with liquid developers having a conductivity offrom about 10 (ohm-cm) to about 10 (ohm-cm)".

An additional liquid development technique is that referred to aswetting development" or selective wetting as described in U.S. Pat. No.3,285,741. In this technique, an aqueous developer uniformly contactsthe entire imaging surface and due to the selected wetting andelectrical properties of the developer substantially only the chargedareas of the imaging surface are wetted by the developer. The developershould be relatively conductive having a resistivity generally fromabout 10 to 10 ohm-cm and having wetting properties such that thewetting angle measured when placed on the imaging surface is smallerthan at the charged area and greater than 90 at the uncharged areas.

While capable of producing satisfactory images, these liquid developmentsystems in general, suffer'deficienci'es in certain areas and are inneed of further development and improvement. Particularly troublesomedifficulties are encountered in liquid development systems employingreusable or cycling electrostatographic imaging surfaces. In thesesystems, for example, a photoconductor such as a selenium or seleniumalloy drum as the photoconductor surface is charged, exposed to a lightand shadow image and developed by bringing the image bearing surfaceinto developing configuration with an applicator containing developingquantities of liquid developer thereon. The liquid developer istransferred according to the appropriate technique from the developerapplicator onto the image bearing surface in image configuration.Thereafter, the developer pattern on the electrostatographic imagingsurface is transferred to copy paper and the liquid developer may beabsorbed by the paper to form a permanent print. During the transferoperation not all the liquid developer is transferred to the copy paperand a considerable quantity remains on the photoconductor surface. Inorder to recycle the imaging surface, this residual developer must beeither removed or its effects immobilized; otherwise it will tend to bepresent as background in subsequent cycles. If the liquid developer isrelatively conductive having, for instance, a resistivity less thanabout 10 ohm centimeters, any residue remaining on the imaging surfacemay dissipate any charge subsequently put on it. Furthermore, lateralconductivity of the liquid developer on the imaging surface may becomeexcessive and the resolution of the resulting image will be poor. Onrepeated cycling, there is also a progressive accumulation of liquiddeveloper on the imaging surface since in each cycle, not all thedeveloper is transferred to the copy paper. This progressiveaccumulation of developer residue results in an overall loss of density,deterioration of fine detail and contributes to increased backgrounddeposits on the final copy particularly since accurate imaging on theimaging surface may by inhibited.

Until recently, the'procedures suggested to be employed to remove thedeveloper liquid from the imaging surface have been so severe andcomplete that there has been a progressive degradation of the imagingsurface particularly a photoconductor surface which degradation lessensits useful life span. The severity of the cleaning step is dictated bythe fact that on cleaning a film from a surface, the film isprogressively split so that on each separate cleaning only aboutone-half the film is removed. The cleaning solvents that have generallybeen suggested as adequate cleaning aids frequently chemically attackthe imaging surface and are hazardous due to their volatility andtoxicity. In some instances, with complete removal of the ink film theelectrical properties of a photoconductor are virtually destroyed by thecleaning operation after only a few cycles. In other instances, thesolvent employed may also act as a solvent for the resin binder in abinder plate or may induce crystallization of a thin layer of a seleniumphotoconductor. These problems have, however, to a very large degree,been solved, or at the very least, minimized by the development of thecycling cleaning system described in copending United States applicationfor Letters Patent Ser. No. 873,103 by Joseph Mammino entitled CleaningSystem, and filed Oct. 31, I969. Therein described is a cleaning systemwherein dry, highly absorbent powders are placed on the imaging surfaceto absorb any residual developer of any particular imaging cycle. Afterabsorbing the developer the fine powders are removed from the imagingsurface.

When the technique described in the above U.S. Application for LettersPatent is applied to an automatic machine concept, difficulties areencountered in handling the dry powder. Since in a machine each cyclemust be completed within a very short period of time between about I to3 seconds, the powder must be quickly applied and removed from theimaging surface. The rapidity of the technique necessary to provide thisresult produces a considerable amount of dust during the cleaning. Thedust is not only present in the cleaning zone which to some extentminimizes the cleaning efficiency with respect to quantity of cleaningpowder employed, but also may be found in other parts of the machinewhere it may lead to contamination of material supply such as developerliquid and paper or may lead to abrasion or fouling of machinecomponents. Furthermore, the quantity of powder employed in thistechnique is relatively large in that the powder is appliedindiscriminately to the entire imaging surface and removed therefrom.While the cleaning powders may be recycled to some extent, means must beprovided to measure the relative remaining absorbent capacity of thepowder, and when the capacity is reduced below a certain level, thepowder must be removed. To minimize or control these problems, thecleaning mechanism may be extremely cumbersome and complicated. It istherefore clear that there is a continuing need for an improved cleaningtechnique in a cycling electrostatographic imaging process.

SUMMARY OF THE INVENTION It is therefore an object of this invention toprovide a developing system which overcomes the above noteddeficiencles.

It is another object of this invention to provide a novel cleaningsystem.

It is another object of this invention to provide a cleaning systemwhich makes more efficient use of cleaning materials.

It is another object of this invention to provide a simple cleaningsystem capable of cyclical use.

It is another object of this invention to provide a cleaning means forcleaning residual liquid developer from a reusable electrostatographicimaging surface.

-It is another object of this invention to provide a simple mechanicalmeans for cyclical cleaning a reusable imaging surface.

It is another object of this invention to provide a liquid developmentsystem superior to known systems.

It is another object of this invention to provide a cleaning systemsuperior to known systems.

The above objects and others are accomplished, generally speaking, byproviding a cycling electrostatographic imaging system having a cleaningsystem which enables complete cleaning of residual liquid developerwithout degradation of the imaging surface. Finely divided dry absorbentcleaning powders are selectively applied to the charged or developedareas of an electrostatographic imaging surface to remove residualliquid developer from the imaging surface in any imaging cycle. Morespecifically, in a liquid development system employing a cycling orreusableelectrostatographic imaging surface the residual developerremaining on the imaging surface in any one cycle is cleaned from thesurface by directly placing the highly absorbent dry powders in theareas on the surface to be cleaned. By so applying the cleaning powdersto the imaging surface, the residual developer is effectively absorbedby the cleaning powder and with a minimum of mixing removes all residualdeveloper on the imaging surface including that which may be present inthe background areas and permits the cleaning powders to be readilyremoved-from the imaging surface.

Any suitable method may be employed to selectively deposit the absorbentcleaning powders on the imaging surface. A particularly preferredtechnique for ease of operation and simplicity of design involvesplacing a charge on the powder so that it may be electrostaticallypulled from a powder applicator to the image or charged area of theimaging surface. To accomplish this result, the cleaning powders must becharged to a polarity opposite that which the electrostatographicimaging surface bears in the image portions.

Any suitable method may be employed to provide cleaning powdersoppositely charged in polarity to the polarity of the image areas of theimaging surface. A particularly preferred technique of great simplicityin design and materials along with excellent functional results is totriboelectrically charge the cleaning powder. The phenomena oftriboelectrification is present when two dissimilar materials arebrought into contact and rubbed against one another with each materialbecoming electrically charged to a polarity opposite to that of theother. The polarity obtained from the triboelectric charging may bedetermined by contacting the two materials, separating them anddetecting the sign of the charge on each with an electrometer. By doingthis with a variety of different materials, one may arrange thematerials in a descending order from positive to negative such that anyone material is negative with respect to any other material above it inthe series. In such a series, materials above a given one listed willdonate electrons to the material and materials below the given one willaccept electrons from the material. Therefore, by appropriatelyselecting materials, the polarity of charge on the cleaning powders maybe regulated.

The finely divided dry, absorbent powders may be triboelectricallycharged in any suitable manner. They may, for example, be brought intocontact with a fur cloth or other solid material. A particularlypreferred manner of charging the cleaning powder is by contacting andtransporting them by means of a granular carrier. In a typicalembodiment, a mass of bead-like solid material and the dry absorbentpowders are agitated to provide the desired charge on the cleaningpowders. The cleaning powders are then transported to the imag ingsurface. They may, for example, be pushed, rolled, or cascaded acrossthe imaging surface. A particularly preferred technique providingoptimum cleaning while conserving materials, machine space andcomponents is to transfer the charged cleaning powder to an applicatorsurface which delivers the cleaning powders to the imaging surface.

The invention may be further illustrated by reference to theaccompanying drawings in which:

FIG. 1 is a schematic view of an embodiment of an electrostatographicimaging system employing the cleaning technique of this invention.

FIG. 2 is a schematic view of a preferred manner of charging,transporting and applying the charged cleaning powder to the imagingsurface.

FIG. 3 is an alternative preferred manner of transporting the chargedcleaning powder from a powder reservoir and charging station to theimaging surface.

In the electrostatographic imaging system depicted in FIG. 1, anelectrostatic latent image is placed on the imaging surface hereillustrated as a rotating cylindrical drum photoconductor 10, such as aselenium drum by uniformly placing a positive charge on the drum bycharging means 12, exposing the charged imaging surface to a light andshadow image through exposure means 11, and developing the electrostaticlatent image at developing station 33. Developing station 33 comprises arotating pattern applicator roll 13 which is loaded with liquiddeveloper 17 by means of feed roller 16 and doctored by doctor blade 14held in place by positioning member 15 to provide liquid developer inthe depressed portions of the applicator surface while the raisedportions are substantially free of developer. The liquid developer 17may be replenished through the developer reservoir 18 by any suitablemeans such as receptacle 19 with a gravity feed to the developer bath18. The developer on the imaging surface in image configuration istransferred to a receiver sheet such as ordinary paper 20, which ismoved through the transfer zone in contact with the drum at the samerate and in the same direction as the periphery of the drum. The paperto which the developed image is transferred is held in transfer positionby idlers 21. Transfer may be electrostatically assisted by means ofcorotron 31. The residual liquid developer present on theelectrostatographic imaging member is then cleaned from the imagingmember at cleaning station 32. At cleaning station 32, a porous,resilient roll 25, containing absorbent charged cleaning powder obtainedfrom bath 24, containing a mixture of cleaning powder 23, and carrierbeads 22, delivers the charged cleaning powder to the imaging surface.The residual developer on the drum 10, may contact the applicator rollcontaining the charged powder, picking the charged powder out of theapplicator in response to the residual charge pattern left on theimaging surface. The residual liquid developer and the charged cleaningpowder on the imaging surface next come in contact with a web-typematerial 27, which is slowly advanced from a supply reel 30, throughidlers 28, into a rubbing contact with the imaging surface and finallyonto takeup reel 26. This cleaning web is preferably moving slowly inthe direction countercurrent to the direction of the advancing imagingsurface so that the cleanest portion of the web contacts the cleanestportion of the imaging surface. In the initial stages of contact betweenthe cleaning web and the imaging surface, the cleaning powder andresidual liquid developer are intimately mixed together and the cleaningpowder substantially and completely absorbs all the liquid forming aliquid loaded powdery residue. The liquid developer loaded powderyresidue is removed from the imaging surface and transferred to thecleaning web. This transfer may be assisted electrostatically by meansof a corotron 29, or any other suitable means such as a biased roll. Asa result of the countercurrent motion of the imaging surface and thecleaning web, the powdery residue is removed from the imaging surface bythe cleanest portion of the cleaning web. The application of charged dryabsorbent powder to an electrostatographic imaging surface and theremoval from the surface of the powder with the absorbed liquiddeveloper provides a substantially complete cleaning of the imagingsurface such that upon passing the cleanest portion of the web cleanerthe imaging surface is prepared for the next imaging cycle.

FIG. 2 is a more descriptive view of the particular cleaning powderapplicator station of FIG. 1. In FIG. 2 the powder applicator device isillustrated as a rotating cylindrical roll which may be independentlydriven or driven by contact with the moving imaging surface. While thisapplicator surface may be moved in a direction concurrent orcountercurrent with the imaging surface, it is preferred to move it in adirection concurrent with the imaging surface to thereby minimize anypossible abrading effect between the powder and the imaging surface andalso to minimize any contamination of either the applicator or thepowder supply by having the liquid developer pushed into it. In otherwords, by moving the applicator surface and the imaging surface incountercurrent directions, a nip of liquid developer would form at theline of contact between the two which would be moving in a directiontoward the reservoir of cleaning powders. In FIG. 2 the powderapplicator here depicted as a cylindrical roll 35, rotates in powderreservoir 36 containing a mixture of clean- 6 ing powders 38, andcarrier beads 37. An additional supply of cleaning powder is containedwithin powder supply chamber 39. A flicker bar 40 is positioned on thereservoir housing so as to serve to contain the carrier beads orcomposite particles in the powder reservoir.

FIG. 3 is an enlarged view of an alternative cleaning powder supplymeans in which the powder applicator surface is depicted as a porousresilient belt 45, driven around support rolls 46, in powder reservoir50, containing finely divided absorbent powder 49, and carrier beads 48.

Any suitable absorbent powder may be employed in the practice of thisinvention. The cleaning aids may generally be described as small sized,highly absorbent, dry powders that function as tiny sponges. To minimizeabrasion the dry powders preferably are softer than the surface whichthey are to clean. For a maximum cleaning effect, the powders are alsopreferably insoluble in the liquid developer, are adhesive after theyhave absorbed the developer liquid thereby enabling their ready removalfrom the surface and further do no introduce anything to contaminate ordegrade the imaging surface. The absorbent powders typically have asurface area of from about 30 to about 9-50 m /gm. Preferably, thepowders have a surface area of from about to about 800 m /gm. Thecleaning powders of this invention may be of any suitable size whichprovides the necessary surface area and absorptive ability. Typically,the particles range in size from about 0.1 to about 20 microns. As thesize of the particles increases, they become more abrasive. On the otherhand, if their size decreases, greater caution must be exercised in thehandling of the powders. For these reasons average particle sizes offrom about I to about 10 microns are preferred with average sizes offrom about 3 microns to about 8 microns providing optimum balance inhandling and performance. The particular cleaning powder should ingeneral be selected based on the absorptive capacity of the powder forthe particular developer. If the developers are oil base material, thecleaning powder preferably has a high oil absorption, if the developeris a polar liquid, the cleaning powder preferably has high absorptivepowers for the polar developer and may have a low oil absorption.Typically, the preferred absorption capacity is from about 40 to about500 milligrams of developer per 100 milligrams of powder.

Typical absorptive microporous materials useful in the practice of thisinvention include the finely divided forms of carbon such as furnaceblack, channel black, lamp black, bone black and charcoal; clays such askaolin and china clay; diatomaceous earth, pumice, fly ash, infusorialearth; pigments such as titanium dioxide, zinc oxide and silica. Typicalorganic pigments having adequate absorptive capacity includequinacridones, phthalocyanines, and benzidine yellow.

Especially satisfactory cleaning is obtained with silica gels, kaolinclays, carbon black, titanium dioxide. Particularly preferred materialsin handling and cleaning ability are the silica gels having an averageparticle size of from about 3 microns to about I 1 microns, a surfacearea of from about 200 m lgm to about 350 m /grn and a liquid absorptionof from about 100 to about 315 mg liquid/100 mg powder.

The absorbing cleaning powder may be applied to the surface to becleaned in any suitable amount. Typically, the absorbing power of thepowder is at least sufficient to absorb all the residual liquid on theimaging surface. Preferably the amount of cleaning powder appliedprovides an absorbing power greater than that necessary to absorb allthe residual liquid. Generally the residual developer is present on theimaging surface in an amount up to a maximum of about 2.3 milligrams persquare centimeter. For such amounts of liquid from about 1 percent toabout 10 percent by weight of the developer of the cleaning powder aregenerally added.

Any suitable applicator means may be employed to transfer the drycleaning powder from a powder supply or reservoir to the imagingsurface. Typical materials would include rollers, webs and brushes. Theapplicator preferably is porous, soft and resilient to thereby hold asufficient amount of dry powder to absorb all the residual developer andto present the dry powder to the imaging surface while minimizing anywear or abrasion due to contact between the two surfaces. A particularlysatisfactory group of materials useful as the powder applicator may bedescribed as porous, cellular foams. Typical cellular foams that may beemployed as a powder applicator include sponge rubber, foamedpolyurethane, polyethylenes, vinyls, polystyrene, cellulose acetate,phenolics, ureas, silicones, epoxies.

The granular carrier material may be of any shape although it ispreferred to have them round or nearly round to facilitate theirmovements in mixing and to provide less friction. Any suitable roundgranular material may be employed. Typical granular materials includesodium chloride, aluminum potassium chloride, Rochelle salt, granularzircon, granular silicone, methyl methacrylate, phenolics, epoxies,ureas, melamines and other resins; glass, silicone dioxide, flint shot,iron, steel, ferrite, nickel, carborundum and suitable mixtures thereof.In

t those instances where the particular granular material does not impartthe necessary triboelectric properties, the granular material may becovered with or encased in a suitable covering which imparts thenecessary triboelectric properties so that it will properly charge thedry cleaning powder when mixed therewith.

To facilitate separation of the dry powder from the granular material,it is preferred that the carrier particles be substantially larger thanthe cleaning particles. Typically, the granular carrier particles have aparticle diameter of between 50 microns to about 600 microns. Thegranular material should be sufficiently large not to be trapped in thecellular structure of the preferred powder applicator layer but shouldnot be so large that it unduly limits the volume of cleaning powder withwhich it may be satisfactorily mixed. For these reasons, the particlediameter of the granular material is preferably between about 150microns and about 450 microns. It is particularly preferred that thegranular carrier material be provided with a lubricating coating toprevent the sticking of one particle to another since the entire mass ofgranular material and dry powders when being agitated should flow like afluid.

To provide uniform charge on the dry cleaning powders, it is preferredthat the granular carrier material and the powder be continuously anduniformly mixed during operation. The granular carrier material and thedry powders may be mixed together in any proportion which will providesufficient charge on a sufficient amount of the dry powders to betransferred to an applicator surface and on subsequent transfer to theimaging surface remove all liquid developer. Typically, the mixture ofgranular carrier and dry powder comprises from about 10 to about 50parts by volume of granular carrier to about 1 part by volume of drypowder. In obtaining uniform charging on the dry powder, therepreferably is from about 20 to about 35 parts by volume granular carrierto 1 part dry powder.

The applicator surface should be sufiiciently porous to hold an adequatequantity of the dry powder to substantially completely absorb all theliquid developer remaining on the imaging surface when pulled from theapplicator to the imaging surface. To assist in transport of the drypowder from the granular carrier to the applicator, it is preferred toprovide an applicator which is capable of being charged to a polarityopposite that of the dry powder. A particularly preferred applicator isa cylindrical roll between about 1 inch to l 1% inch in diametercomprised of a steel core with an adhesive backed polyurethane spongecemented around it.

Since the cleaning powders and techniques of this invention provide asubstantially complete cleaning of residual developer from the imagingsurface on every cycle, no problem of charging and imaging through aresidual liquid film exists and developers of both low and highconductivity may be employed. Any suitable developer may be used.Typically, the developers for which the dry absorbent powders of thisinvention are effective have a conductivity of from about 10 (ohm-cm) toabout l -(ohm-cm) Typical vehicles within this group providing theseproperties include glycerol, polypropylene glycol, 2,5 hexanediol,mineral oil, the vegetable oils including castor oil, peanut oil,coconut oil, sunflower seed oil, corn oil, rapeseed oil, sesame oil.Also included are mineral spirits, fluorinated hydrocarbon oil such asduPonts Freon solvents and Krytox oils; silicone oils, fatty acidesters, kerosene, decane, toluene and oleic acid. in addition, as iswell known in the art, the developers may contain one or more secondaryvehicles, dispersants, pigments or dyes, viscosity controlling agents oradditives which contribute to fixing the pigment on the copy paper.

Any suitable electrostatographic imaging surface may be cleaned throughthe technique of this invention. Basically any surface upon which anelectrostatic charge pattern may be cyclically formed or developed maybe employed. Typical electrostatographic imaging surfaces includedielectrics such as plastic coated papers, xero printing master,photoconductors and overcoated photoconductors. Typical photoconductorsthat may be employed include selenium and selenium alloys, cadmiumsulfide, cadmium sulfoselenide, phthalocyanine binder coatings,polyvinyl carbazole sensitized with 2,4,7 trinitrofluorenone. Typicalovercoated photoconductors include those described in US. Pat. Nos.3,234,019 and 3,25 1,686. A specific example of an overcoatedphotoconductor is a selenium layer on a conductive aluminum substrateovercoated with a thin film of polyethylene terephthalate. Theelectrostatographic imaging surface may be employed in any suitablestructure including plates, belts, or drums may be employed in the formof a binder layer. For more effective cleaning, it is preferred toprovide a surface to be cleaned which has a very smooth surface andwhich is non-absorbent for the liquid developer since generally the moresmooth and uniform the surface, the better will be the cleaning.

After the dry, absorbent powders are applied to the imaging surface, itis generally preferred to distribute the cleaning powder over theimaging surface so that the entire surface is contacted with the dryabsorbent powders to thereby insure maximum absorption of all residualliquid developer in the imaging surface including the residual developerin the image or developed areas and those that may be present in thebackground or undeveloped areas. When the dry absorbent powders employedin this invention have absorbed all the residual liquid developer, theyare in the form of a solid powdery residue containing the absorbeddeveloper within the powder and they must be removed from the imagingsurface prior to the next imaging cycle. Any suitable cleaning systemmay be employed. Typical cleaning systems include: wiper blades,brushes, vacuum suction and cleaning webs. Particularly satisfactorydistributing of cleaning powder and cleaning is obtained with the use ofa fibrous cleaning web moving countercurrent to the direction ofmovement of the imaging surface such that the cleanest portion of theimaging surface is contacted and finally cleaned by the cleanest portionof the cleaning surface. The cleaning web may be made of any suitablematerial. Typical fibrous cleaning webs include those made fromcheesecloth, flannel, rayon, cotton, and combinations of rayon andcotton, Dacron, polyester and/or polypropylene fibers. Particularlysatisfactory cleaning is obtained with those fibrous webs which aresubstantially homogeneous and thick and have a high absorbent capacity.As indicated in FIG. 1 of the drawing, transfer of the dry powdercontaining absorbed liquid developer to the absorbent web may befacilitated by electrostatic transfer from the im aging surface to theweb. This may be accomplished by the use of a corotron or a biasedmember disposed on the side of the cleaning web opposite the imagingsurface.

in operation, an electrostatic latent image is placed on anelectrostatographic imaging surface in a conventional manner. Theelectrostatic latent image is thereafter developed with a liquiddeveloper according to any of the techniques previously discussed.Development preferably is obtained with the use of a patterned surfaceapplicator roller wherein the liquid developer is present in thedepressed portions of the applicator, while the raised portions aresubstantially free of developer and the developer is pulled from thedeveloper applicator to the imaging surface in image configuration.After transfer of the developer from the imaging surface to receiversheet in image configuration the residual developer remaining on theimaging surface is removed from the imaging surface according to thetechnique of this invention. According to this technique, andparticularly according to the preferred embodiments illustrated in FIGS.1 and 2 of the drawing, a mixture of granular carrier material and dryabsorbent powder is provided and agitated to apply a charge to thecleaning powder. Thereafter, the powder is separated from the carrierand transferred to a porous resilient applicator which in turn ispositioned close to or in contact with the imaging surface where thepowder is transferred to the imaging surface in image configuration.Preferably the powder is separated from the granular carrier material byapplying a charge of polarity opposite that as the powder to theapplicator. In aparticular example employing a selenium photoconductoras the electrostatographic imaging surface, the selenium would bepositively charged and the image areas would remain positively chargedfollowing exposure. To be electrostatically attracted to the image areasof the selenium imaging surface, the dry absorbent powder should benegatively charged. Accordingly, the powder applicator is positivelycharged to pull the dry powder free from the granular carrier and thegranular carrier also has acquired a positive charge due to thetriboelectric contact between the granular carrier and dry powder. Thepolarity of charge placed on the granular carrier by the triboelectriccontact and the polarity of any charge placed on the powder applicatormust, therefore, be the same as the polarity of charge on the imagingsurface. However, to provide the necessary transfer of dry powder fromcontact with the granular carrier to the imaging surface, the magnitudeof potential on the powder applicator preferably is greater than thepotential on the granular carrier and the residual potential on theimaging surface is greater than that on the powder applicator. However,while electrostatic transfer may be a principal means of transfer of drypowder from carrier to powder applicator and from a powder applicator tothe imaging surface it is not the sole means of transfer. The powder maybe transferred to the porous applicator from the carrier by physicalscraping during contact. A considerable force acting towards transfer tothe imaging surface is the adhesive nature of the liquid residue on theimaging surface. It is the combination of electrostatic transfer and theadhesive nature of this liquid which provide the necessary transfer ofthe dry cleaning particles to the imaging surface. While the powderapplicator .and the imaging surface may come in contact, it is preferredthat they do not come in contact so that any residual developer on theimaging surface does not offset onto the powder applicator. Residualdeveloper remaining on the imaging surface builds up to a thickness ofapproximately 3 microns. Therefore, in order to minimize any possibleoffset to the powder applicator, it is preferred that a layer of dryabsorbent powder be provided on the powder applicator in excess of 3microns and preferably from about 5 to about microns in thickness. Sucha relatively thick layer of powder also facilitates adhesive transfer tothe imaging surface. Once the dry absorbent powder has been placed onthe imaging surface to provide the maximum cleaning efficiency, it ispreferred to uniformly distribute the dry powder over the imagingsurface to absorb all the liquid developer remaining on that surface.This may be accomplished through any suitable means such as the cleaningweb depicted in FIG. 1. With a cleaning web moving in a directioncountercurrent to the direction in which the imaging surface is movedthe dirtiest portion of the cleaning web would provide this smearing ordistributing function in which all of the liquid developer would beabsorbed by the absorbent powders. Further upstream in the arch formedby the contact between the cleaning web and the imaging surface, thecleaning web will remove all the absorbent powders which, while theyhave absorbed the liquid developer, remain powdery and substantiallydry. The rate at which a web of cleaning material is consumed is afunction of the rate of plate movement and the relative rate of the webrequired to yield satisfactory cleaning and has been found to vary tosome degree dependent upon the particular cleaning material employed.Typically, the cleaning web is found to have a speed on the order ofone-fortieth to one four-hundredth of the plate speed.

While the cleaning web and imaging surface may be moved in the samedirection, minimum contact length has been found to occur when the weband plate are moved in substantially opposite directions.

The absorbent powder applicator may be independently driven or driven bythe imaging surface. Preferably, it will be driven in the same directionas the imaging surface since if driven in a direction opposite to thatof the imaging surface, the liquid developer would tend to fill the nipbetween the two surfaces and serve to contaminate the roll at this nip.When the absorbent powder applicator is frictionally driven by theimaging surface, it is in contact with the imaging surface under theminimal driving pressure necessary to provide adequate motion. Ifexcessive pressure is employed, the absorbent powder may be mechanicallypressed onto the entire imaging surface from the powder applicator thususing more powder than necessary and requiring additional cleaningeffort later to remove this dry powder. The absorbent powder applicatormay be driven at the same speed or a speed slightly slower or fasterthan the imaging surface provided the abrading effect is minimized andthere is no transfer of liquid developer to contaminate the powderapplicator.

The agitation of the granular carrier material and the absorbent powdersshould be sufficient to provide uniform mixing and charging of theabsorbent powders and preferably takes place in an area adjacent to andin contact with the powder applicator. Providing this type of mixing incontact with a portion of the powder applicator minimizes any possibleclogging of the applicator roller by some wetted powders which may bepresent on the powder applicator. By providing a sufficient mixing, thewetted powders should be removed from the applicator surface anduniformly distributed throughout the body of the cleaning material. Thisagitation may be provided simply by the stirring action of a powderapplicator roller such as that depicted in FIG. 2. Any suitable means,however, may be employed such as a vibrating or stirring means. Anypowder accumulating on the powder applicator that is undesirable may beremoved from the applicator surface by positioning a flicking bar orpinch member in contact with the applicator.

DESCRIPTION OF PREFERRED EMBODIMENT The following preferred examplefurther defines and describes preferred materials, methods, andtechniques of the present invention. In the example, all parts andpercentages are by weight unless otherwise specified.

EXAMPLE I An imaging system similar in configuration to that depicted inFIG. 1 is assembled. A photoconductor in the form of a drum comprising asurface layer of selenium about 50 microns thick on a conductive plateis positively charged to about 450 volts and exposed to a light andshadow image in conventional manner. The electrostatic latent image thusformed is developed by moving a patterned surface applicator roll havingdeveloping quantities of a developer in the depressed portions thereofpast the image bearing surface so that liquid developer is pulled out ofthe depressed portions to the image bearing surface in imageconfiguration. The speed of development is about 10 inches per second.The developer employed is of the following composition by weight:

Drakeol 9 30 parts by weight Microlith CT [8 parts by weight Methylviolet tannate 3 parts by weight Ganex V2l6 l5 pans by weight ParaflintRG wax 0.5 pans by weight Drakeol 9 is a mineral oil manufactured byPennsylvania Refining Company having a kinematic viscosity of about 15.718.1 centistokes at 25C. and a specific gravity of about 0.85. MicrolithCT is a resinated predispersed carbon black pigment composed of about 40percent carbon black pigment and 60 percent ester gum resin,manufactured by CIBA. Paraflint R6 is a hard synthetic wax available inflake form from Moore & Munger. Ganex V216 is an alkylated polyvinylpyrrolidone compound manufactured by GAF Corporation which serves asadditional pigment dispersant and may also be regarded as a secondaryvehicle.

The developer on the photoconductor is transferred to bond paper inimage configuration. A reservoir containing about cubic centimeters ofSyloid 308, a silica gel available from Davison Chemical Division, W. R.Grace & Company, and about 175 cubic centimeters of about 450 microndiameter steel spheres. A sponge roll of about 1% inches in diametercomposed of a polyurethane sponge containing about 80 pores perlinearinch on the surface, available from Scott Paper Company under the nameof Scott Industrial Foam, and about one-eighth inch in thickness wrappedaround a 1 inch aluminum core is rotated in the reservoir so that thelower portion is below the surface of the mixture of steel spheres andsilica gel. The polyurethane powder applicator roll is driven by slightcontact with the selenium drum at a speed of about inches per second. Arayon web Miracloth, code 9243 manufactured by Chickapee Mills isadvanced in a direction opposite to the rotating selenium drum along anarc of the drum surface of about 5 inches at a speed of about oneone-hundredth that of the drum speed. The first print obtained with thisapparatus is free of background and has a resolution of about 10 linepairs per millimeter. After repeated cycling of 1000 prints, nosignificant change in print quality is observed. Very little dusting andno fouling of the mechanical movements by any of the cleaning powder areobserved.

The technique provided by the instant invention provides a substantiallycomplete cleaning of all residual liquid developer from an imagingsurface without any significant abrasion of the imaging surface and avery fast and efficient manner and employs very few mechanical movingparts. It further provides a cleaning system capable of cyclical usewhich minimizes dusting and conserves expendable material by applyingonly sufficient cleaning powder to the areas to be cleaned. in addition,the final material being handled after the cleaning has beenaccomplished is a substantially powdery residue.

Although specific materials and operational techniques are set forth inthe above exemplary embodiments using the cleaning technique of thisinvention, these are merely intended as illustrations of the presentinvention. There are other materials and techniques than those listedabove, which may be substituted with similar results.

Other modifications of the present invention will occur to those skilledin the art upon a reading of the present disclosure which modificationsare intended to be included within the scope of this invention.

What is claimed is:

1. The method of cyclically developing electrostatic latent images on areusable electrostatographic imaging surface comprising the steps offorming an electrostatic latent image on the imaging surface, developingthe image with a liquid developer, transferring the developer from theimaging surface to the receiving surface in image configuration,selectively applying dry absorbent cleaning powder to the image area ofsaid imaging surface to absorb substantially all of the liquid developeron the imaging surface and removing the absorbent powders containingabsorbed liquid to prepare the imaging surface for the next imagingcycle and repeating the recited steps of forming, developing,transferring, applying and removing at least one additional time.

2. The method of claim 1 wherein said dry powder has a charge oppositein polarity to the charge in the image areas of the ima ing surface.

3. Tfie method of claim 1 wherein said dry absorbent powder istriboelectrically charged by contact with a granular carrier material.

4. The method of claim 1 wherein the imaging member is a reusablephotoconductor.

5. The method of claim 1 wherein the photoconductor is selected from thegroup consisting of selenium and selenium alloys.

6. The method of claim 1 wherein said selective application of absorbentpowder is electrostatically assisted.

7. The method of claim 1 wherein the imaging member is treated with anamount of absorbent powder at least sufficient to absorb substantiallyall the liquid on the imaging member.

8. The method of claim 1 wherein the absorbent powder is substantiallyinsoluble in the liquid developer.

9. The method of claim 1 wherein the powder has a surface area of fromabout 100 to about 800 m lgm.

10. The method of claim 1 wherein the powder has a particle size of fromabout i to about 20 microns.

11. The method of claim 1 wherein said powder is removed from saidimaging member by contacting the imaging member with an absorbentfibrous material.

12. The method of claim 1 wherein said absorbent powder is a silica gel.

13. The method of claim 1 wherein said absorbent powder is a finelydivided carbonaceous material.

14. The method of claim 1 wherein said carbonaceous material is carbonblack.

2. The method of claim 1 wherein said dry powder has a charge oppositein polarity to the charge in the image areas of the imaging surface. 3.The method of claim 1 wherein said dry absorbent powder istriboelectrically charged by contact with a granular carrier material.4. The method of claim 1 wherein the imaging member is a reusablephotoconductor.
 5. The method of claim 1 wherein the photoconductor isselected from the group consisting of selenium and selenium alloys. 6.The method of claim 1 wherein said selective application of absorbentpowder is electrostatically assisted.
 7. The method of claim 1 whereinthe imaging member is treated with an amount of absorbent powder atleast sufficient to absorb substantially all the liquid on the imagingmember.
 8. The method of claim 1 wherein the absorbent powder issubstantially insoluble in the liquid developer.
 9. The method of claim1 wherein the powder has a surface area of from about 100 to about 800m2/gm.
 10. The method of claim 1 wherein the powder has a particle sizeof from about 1 to about 20 microns.
 11. The method of claim 1 whereinsaid powder is removed from said imaging member by contacting theimaging member with an absorbent fibrous material.
 12. The method ofclaim 1 wherein said absorbent powder is a silica gel.
 13. The method ofclaim 1 wherein said absorbent powder is a finely divided carbonaceousmaterial.
 14. The method of claim 1 wherein said carbonaceous materialis carbon black.